Process for relieving residual stresses in metals



Nov. 1, 1966 A. H. HOLTZMAN 3,

PROCESS FOR RELIEVING RESIDUAL STRESSES IN METALS Filed April 22, 1964 2She ets-Sheet 1 FIGl Nov. 1, 1966 A. H. HOLTZMAN 3,282,743

PROCESS FOR RELIEVING RESIDUAL STRESSES IN METALS Filed April 22, 1964 2Sheets-Sheet 2 INVENTOR ARNOLD H. HOLTZMAN ATTORNEY United States Patent3,282,743 PROCESS FOR RELIEVING RESIDUAL STRESSES IN METALS Arnold H.Holtzman, Cherry Hill Township, Gloucester County, N.J., assignor to E.I. du Pont de Nemours and Company, Wilmington, Del., a corporation ofDelaware Filed Apr. 22, 1964, Ser. No. 363,058 6 Claims. (Cl. 1484) Thisapplication is a continuation-in-part of my copending application SerialNo. 171,184, filed February 5, 1962, now abandoned.

The present invention relates to a process for relieving residualstresses in metals.

Residual stresses, or internal stresses, are produced in metals by anyoperation which brings about a nonuniform change in shape or volumethroughout a Work-piece. Such a change can be effected by heat treating,quenching, Welding, casting, forming, machining, grinding, plating, andother operations that cause local plastic flow. The stresses may developdirectly by expansion or contraction, and may also result from changesin volume and coefiicient of expansion that accompany metallurgicalphase transformations.

Residual stresses often have a deleterious effect on the mechanicalproperties of a workpiece leading to service failure and, moreover, arefrequently the cause for the occurrence of undesirable dimensionalchanges in the piece upon machining. Such stresses can cause metals tocrack quickly when exposed to certain atmospheres, to liquids(stress-corrosion cracking), or to liquid metals; or when heated(fire-cracking), aged (stress-precipitation cracking), or cut into.Residual stresses can also cause metals to warp when machined or cutwith resultant secondary efiects such as binding and breaking ofreamers. Obviously, since residual stresses may affect safety orutility, it is important that the magnitude of these stresses bereduced, i.e., that the metal be stress-relieved. This reduction ofstress level ordinarily is accomplished thermally or mechanically, or bycombined mechanical and thermal means. In thermal stress-relieving, thetemperature of a metal object is increased to a suitable value and thattemperature maintained for a suitable time. Since most high residualstresses are produced by thermal gradients caused by too rapid or unevencooling, with attendant plastic flow, it is apparent that such stressescan be lowered or removed if the structure is properly reheated and thencooled slowly and evenly. However, because of the conditions re quiredfor thermal stress relief, the procedure cannot be used conveniently oreffectively with all types of materials and with pieces of any size andshape. For example, in the thermal stress-relieving of large parts, theneed for large furnaces and high heat inputs is an economicdisadvantage, as is the complex system of temperature control which isnecessary to insure that the thermal input is distributed evenly overthe receiving surfaces. The heating of large parts in sections orprogressively by passage of the piece through a furnace requires carefulcontrol to avoid insuflicient overlapping of successive steps or anexcessive rate of movement, and to protect the section adjacent to theheated zone from serious temperature gradients.

In addition to the difiiculties encountered in thermallystress-relieving large parts, this method has limitations also withrespect to resulting adverse effects on mechanical properties in certainmetals since the thermal cycles used sometimes produce undesirablechanges, either microstructural or of other kinds. The adverse effect ofthermal stressrelief procedures on mechanical properties is especiallypronounced in the case of the precipitation harden- 3,282,743 PatentedNov. 1, 1966 "ice ing alloys, e.g., certain alloys of aluminum, copper,magnesium, or nickel, and the steels. For example, certain aluminumalloys are precipitation hardened, i.e., age-hardened, by heating atabout 300 F. after heat treatment, to produce a definite microstructureand definite mechanical properties. Temperatures of the order of 450 to500 F. are required for producing substantial relief of residualstresses in these alloys in a reasonable length of time, but thesetemperatures destroy the effects of the previous aging treatment.

As an alternative to thermal stress relief, mechanical methods employingmoderate temperatures have been used to reduce residual stresses.Mechanical methods, e.g., hand peening, shot peening, pressing, rolling,and stretching, usually are applied to machines or structures whicheither are too large to be placed in a furnace, or of such a shape thatthermal stress relief would cause excessive distortion. Althoughmechanical methods involve less expensive equipment, they are seldom aseffective as thermal methods in that they ordinarily produce onlysuperficial stress relief, and also they generally require extensivedeformation of the piece. Cold compression treatment, for example,results in considerable compressive deformation and, when applied tocomplex shapes, requires an investment in dies.

The use of hydrostatic pressure to reduce stresses in metal objects alsohas been proposed but the procedure is cumbersome and particularlydiflicult when large parts are to be treated, requiring large containersof high wallstrength to resist the hydrostatic pressures used. Moreimportantly, the gradually applied pressure is incapable of effecting adeep permanent stress relief.

Since the relieving of residual stresses in metals is of vitalimportance in the metallurgical art owing to the danger involved in theuse of metal structures containing such stresses, a method whicheffectively relieves stress and, at the same time, does not possess thedisadvantageous features of the heretofore-known methods is extremelydesirable and would advance the art considerably.

It now has been found that residual stresses in metals can be relievedby subjecting the metal to the action of an explosively inducedlow-intensity transient pressure wave, for example, a shock wave.

In accordance with the process of the present invention, a metal objectto be stress relieved is positioned in spaced relationship to adetonating explosive; a shock wave transfer medium is placed between theexplosive and the metal object to provide a buffer zone therebetween;and the explosive is detonated, the pressure wave resulting fromdetonation of the explosive and traversing the metal object, or at leastthat portion of the object to be stress relieved, having an intensitywhich exceeds, but is less than five times, the Hugoniot elastic limitof the metal object. Preferably, the metal object to be treated isspaced apart from a layer of detonating explosive, preferably conformingto the general contour of the object, and the space therebetween isfilled with a liquid, preferably water.

The term shock wave transfer medium is used herein to denote a materialor substance which occupies the buffer zone, that is, the space betweenthe explosive and the metal object being treated, which material orsubstance is capable of attenuating or partially absorbing the pressurewave, e.g., shock Wave, produced by detonation of the explosive anddirected toward the metal object to be stress relieved.

The term Hugoniot elastic limit is used herein in the conventional senseto denote the intensity of a pressure wave in a material at the dynamicelastic limit of the material. The elastic limit of a material can bedefined as the maximum stress that can be applied to a body of thematerial after the removal of which the body is able to regain itsoriginal dimensions. When the stress is applied by shock wavetechniques, the elastic limit becomes the dynamic elastic limit, whichis a point at which a discontinuity occurs in the pressure vs. specificvolume curve for the materal (commonly known as the Hugoniot curve),this discontinuity indicating that a marked change has occurred in thecompressibility of the material. As a consequence, the dynamic elasticlimit is referred to more specifically as the Hugoniot elastic limit.This property is described in detail and values are given for a varietyof materials in Response of Metals to High Velocity Deformation, editedby P. G. Shewmon and V. F. Zackay, New York, Intcrscience, 1961; pp.165-203, G. E. Duvall, Some Properties and Applications of Shock Waves;and pp. 249-274, F. S. Minshall, The Dynamic Response of Iron and IronAlloys to Shock Waves.

In the present process, detonation of the explosive causes a shock waveto enter the shock-wave-transfer medium. Under some conditions, i.e.,when the detonation velocity of the explosive is sufliciently highrelative to the speed of sound in the metal object to bestress-relieved, the pressure wave traversing the metal object also is ashock wave, i.e., travels faster than sound in the metal. In such cases,the pressure of the shock wave in the metal object under a specific setof conditions, i.e., type and amount of explosive charge, distancebetween the charge and the metal object, and density of theshockwave-transfer medium, can be determined by measuring the shock wavevelocity in the object and the velocity with which the free surface ofthe object moves when the shock wave reaches that surface (free-surfaceor particle velocity). A number of techniques are known for measuringshock wave velocity and free-surface velocity, any one of which can beused. Such techniques are described in detail in Modern Very HighPressure Techniques, R. H. Wentorf, Jr., editor, Washington,Butterworths, 1962; chapter 11, by W. E. Deal, Jr.: DynamicHigh-Pressure Techniques. These include, for example, optical methodsusing a smear or streak camera, or a framing camera, and electricalmethods such as the pin or condenser method. Multiple-flash X-rayphotographs also can be employed. The pressure, P, of the shock wave inthe metal object is calculated from the equation:

P P U U when p is the initial density of the metal, and U and U are theshock velocity therein and the particle velocity, respectively. In theabove equation, density, shock velocity, and particle velocity in e.g.s.units give the pressure in dynes/cm. Pressure in kilobars is obtained bydividing the pressure in dynes/cm. by

When the sonic velocity of the metal object is high relative to thedetonation velocity of the explosive used, the transient pressuredisturbance effected in the metal object may not be a shock wave. Insuch a case, the pressure of the wave in the metal object can bedetermined by finding the pressure of the shock wave in theshockwavetransfer medium adjacent the metal surface by methods describedabove, and multiplying this value by 2, the pressure in the metal objectbeing approximately double that in the adjacent medium, which has alower sonic impedance than the metal object.

In the preferred embodiment of the present process wherein the spacebetween the metal object and the explosive is filled with water, analternative method can be used to determine the pressure of the wave inthe metal. The velocity of the shock wave can be measured at thewater-metal interface, e.g., by use of a framing camera (such as theBeckman and Whitley model 189), and the pressure of the shock wave canbe found by referring to Underwater Explosives, by Robert H. Cole,Princeton University Press, 1948, page 40, FIGURE 2.3, whereinvelocities at a shock front in water are plotted as a function ofpressure for high pressures. The pressure of the wave in the metalobject is approximately twice the shock pressure in the water at theinterface.

The Hugoniot elastic limit of a metal can be determined by a study suchas that carried out by Minshall, above cited. The Hugoniot equation ofstate of the metal is studied by using the pin or capacitor technique(electrical methods) to measure the velocities of elastic and plasticwaves in the metal and the velocities with which the free surface moveswhen these waves reach that surface and are successively reflected fromit. The pressure at the Hugoniot elastic limit, p is given by:

wherein p =1/V is the initial density of the metal, U is the velocity ofthe Hugoniot elastic wave, and u is the particle velocity.

In order to describe the invention more fully, reference is now made tothe accompanying drawings, which illustrate suitable embodiments.

FIGURE 1 shows a perspective view of an assembly for stress-relieving aflat metal object by the process of the present invention;

FIGURE 2 depicts an assembly for stress-relieving a cylindrical metalobject by the present process;

FIGURE 3 depicts an assembly for stress-relieving a metal object ofirregular contour;

FIGURE 4 shows a top view in detail of the metal object used in theassembly of FIGURE 3, said object having both planar and curved surfacesand nonuniform thickness, i.e., having flange and web portions;

FIGURE 5 shows a cross-section of the object shown in FIGURE 4 togetherwith superposed buifer, e.g., water, and explosive layers; and

FIGURE 6 shows a different cross-section of the same object togetherwith superposed buffer, e.g., water, and explosive layers.

In all figures, identical elements are indicated by identical symbols.

In FIGURE 1, the metal object 1 to be treated, in the form of a plate,rests at the bottom of vessel 4. A layer of detonating explosive 2 ispositioned above metal plate 1 in such a way that the horizontalsurfaces of the explosive layer and the metal plate lie in parallelplanes and the vertical surfaces are essentially aligned. The explosivelayer 2 is held in its desired position in vessel 4 by any convenientmeans (not shown), e.g., supported on a thin piece of wood which fitssnugly in place in vessel 4. A shock-wave-transfer medium 3, in thiscase water, is present in vessel 4 to a depth suflicient to fill thespace between metal plate 1 and explosive layer 2 completely, forming abuffer layer therebetween. Explosive layer 2 is initiated by line-wavegenerator 8, which is actuated by any standard initiator e.g., anelectric blasting cap 9, the lead wires 7 of which extend to a source ofelectricity (not shown).

Detonation of the line-wave generator 8 produces a detonation frontwhich arrives simultaneously at a number of points along the adjoiningedge of explosive layer 2, producing therein a straight-line detonationtraveling through layer 2. Detonation of layer 2 induces a shock Wave inthe water layer 3, which traverses the water 3 and causes a transientpressure disturbance in the metal 1 having an intensity greater than,but less than about five times, the Hugoniot elastic limit of the platemetal measured as previously described.

FIGURE 2 illustrates in cross-sectional view an assembly forstress-relieving a metal object 1, which in this case is a solid steelcylinder. The explosive layer 2 and sh0ck-wavetransfer medium 3 in thisembodiment completely surround the longitudinal surface of cylindricalobject 1 in a substantially uniform relationship to provide essentiallyconsistent relief of internal stress throughout the cylinder. Twoline-wave generators 8 and 8 are attached to explosive layer 2 and againany standard initiator, e.g., electric blasting caps such as shown inFIG- URE 1 may be attached to the line-wave generators 8 and 8 toinitiate them. The buffer layer of a shockwave-transfer medium 3, i.e.,water, extends laterally the length of the cylindrical object 1 and isof substantially uniform width throughout. The cylindrical object 1 inFIGURE 2 is seen to be positioned in vessel 4 in such a way that itrests on one of its planar surfaces; however, any suitable holding meanscould be used to affix the cylinder and the explosive layer in a desiredposition.

FIGURE 3 illustrates an assembly for stress-relieving a metal object 1of irregular contour. Water 3 is present as the shock-wave transfermedium and explosive layer 2 is positioned over the water 3 and metalobject 1 so that the horizontal plane of the explosive layer 2 isparallel to the horizontal axis of the metal object 1. The verticalsurfaces of the explosive layer and the metal object are not ali ned atall points, i.e., the explosive layer does not have exactly the samecurved contour as the metal object, although the explosive layer 2overlies all points on the horizontal surface of the metal object 1.

The metal object 1 shown in the assembly of FIGURE 3 is shown in greaterdetail in top view in FIGURE 4. The metal object is a cavity die forginghaving a combination of curved surfaces and planar surfaces. As used inthe assembly of FIGURE 3, however, the object shown in FIGURE 4 may beconsidered as essentially a fiat object of uneven surf-ace. The forginghas flange portions in and web portion 11); i.e., the outermost portion1a is of greater thickness than the innermost portion 1b.

The cross-sections shown in FIGURES 5 and 6 show the metal object ofFIGURE 4 together with superposed water and explosive layer. Zia and 1bagain indicate the flange portions and web portion, respectively, 2 theexplosive layer, and 3 the water buffer layer. The area between flangeportions la on the side opposite to the explosive is filled with asupport material 10, preferably a metal, to prevent distortion of webportion 112 during treatment, and the water 3 is seen to completelyoccupy the space between explosive layer 2 and metal object 1.

The process of the invention provides a means of achieving a substantialreduction in residual stresses in metals without adverse effects on themechanical properties or shape of the workpiece and without the need forexpensive equipment such as large furnaces, large tanks withpressure-resistant walls, temperature-control devices, or dies. As isshown in the accompanying drawings and in the following examples, theprocess can be used to relieve stresses in metal objects of widelydifferent shapes and degree of uniformity. In addition to the fiat metalobject, the forging, and the cylinder shown (FIGURES l, 3, and 2,respectively), the metal to be stress-relieved may be in the form of arod, disk, or the like, such as any of the possible configurationsformed by the conventional metallurgical operations of casting,extrusion, sheeting, forging, etc.; and unsymmetrical as well assymmetrical samples can be treated since the exact configuration of thesample is not critical.

A critical feature of the process of this invention is the passagethrough the stress-containing metal object of a transient pressuredisturbance of an intensity which exceeds the Hugoniot elastic limit ofthe metal but is less than about five times this limit. Below this limitthe metal behaves elastically and a permanent relief of stresses doesnot occur. When the transient pressure is excessive, i.e., when thispressure is about five or more times the Hugoniot elastic limit for themetal, useful stress relief is not produced, e.g., stresses may beincreased or the piece may be distorted. Values of the Hugoniot elasticlimit for various metals can be found in Response of Metals to HighVelocity Deformation, supra, particularly on pages 193, 259, 270, and271. As is seen therein, the Hugoniot elastic limit of an unannealedaluminum is 5.4 kilobars and that of the same aluminum annealed is 0.9kilobar. Thus, for relieving stresses in aluminum and aluminum alloysthe present process employs a shock pressure in the metal of about 1-30kilobars. Most of the steels are seen to have a Hugoniot elastic limitof l0- l2 kilobars, with those having received special treatment havinghigher limits. Thus, for steels the present process can employ a shockpressure of about 10-60 kilobars, although pressures greater than about50 kilobars generally are not preferred to assure that additionalstresses are not introduced into the metal object.

The pressure wave of an intensity within the abovespecified range isintroduced into the metal object containing residual stresses bydetonating an explosive adjacent, and preferably in contact with, ashock-wave-transfer medium which in turn is adjacent, and preferably incontact with, the metal object and which separates the metal object fromthe explosive. The transient pressure in the metal is controlled, interalia, by the amount and detonation velocity of the explosive used, thedensity of the shock-wave-transfer medium, and the distance between theexplosive and the metal object. To provide suificient mass between theexplosive layer and the metal object for the necessary shock attenuationwithout the need for an excessively large volume of theshock-wave-transfer medium, the latter preferably will have a density ofat least about one gram per cubic centimeter. While the density of theshock wave-transfer medium can vary greatly, the sonic impedance of themedium should be less than that of the metal object to assure therequired pressure in the object. Gases are not suitable as the solebuffer medium since they do not provide sufficient mass and also becausethe high temperatures resulting from shocks in gases may have adeleterious effect on the properties of the metal object. Thus, theshock-wave-transfer medium can be a liquid or a somewhat compressiblesolid. A certain degree of compressibility is required in order for theshock pressure to be reduced as a result of compressive work. On thebasis of economy, ease of use, and property requirements, liquids arethe preferred shock-wave-transfer media, especially aqueous media.Solids which can be used are, for example, rubber, foamed polymers,e.g., polystyrene foam, and metals, e.g., lead.

The shock-wave-transfer medium has a surface adjacent the explosive andan opposite surface adjacent the metal object containing stresses andseparates the metal object from the explosive. Upon initiation of theexplosive, a shock Wave of a specific pressure enters the buffer layer,travels through this layer, and causes a transient pressure disturbanceof less intensity in the metal object. Usually, the buffer layersubstantially completely fills the space between the explosive and themetal object so that it is not thrown against the metal object by theaction of the shock wave, a condition which usually is to be avoided inorder to assure shock attenuation.

Greater shock attenuation is achieved with denser and thickerbuiferlayers. Therefore for a specific combination of explosive compositionand loading, and metal of a specific Hugoniot elastic limit, a thinnerbuffer layer can be employed with a denser shock-wave-transfer mediumthan with a lower-density material to achieve a desired pressure. Theparticular thickness of buffer layer which will be used in any specificcase will depend on how much attenuation is needed (i.e., on thepressure required in the metal with respect to the pressure of the shockwave produced at the explosive-buffer interface) and the density of theattenuating material. As is illustrated in Example 1, buffer layerthicknesses of one-quarter, one-half, and one inch are equallysatisfactory when Water is used as the shock-wave-transfer medium withan explosive layer detonating at about 7000 meters per second and at aloading of 2 grams per square inch. As

a general rule, the buffer layer should be at least one-six teenth of aninch thick. The maximum thickness of the buffer layer cannot be readilyfixed on the basis of operability inasmuch as an extremely large spacingbetween the explosive and the metal object can generally be compensatedfor by a higher explosive loading.

In the process of the invention the pressure wave introduced into thestress-containing metal object is produced as a result of the detonationof an explosive. To obtain the desired stress relief, the explosive usedmust be a high or detonating explosive, that is one whose reaction rateexceeds the velocity of sound therein, or one having a reaction rate ofat least about 1200 meters per second. The shape of the explosive is notcritical, e.g., it can be spherical or in the form of a layer of anydesired size and configuration, provided that the intensity of thepressure wave introduced into the metal object is relatively uniformalong the entire surface at which the wave enters the object. Uniformintensity is desirable to prevent distortion of the metal object. Forbetter uniformity, the explosive preferably is in the form of a uniformlayer which can be positioned substantially parallel to the metalobject. However, a point explosive charge can be used if the charge issufficiently separated from the metal object so that the pressure alongthe wave front is relatively uniform 'when the wave reaches the metalobject. With point charges the explosive will be separated from themetal object by a distance of several feet or more; with an explosivelayer, there is no advantage to having a separation of more than about afoot.

The exemplified compositions, which detonate at high velocity, i.e., ofthe order of 5000-7000 meters per second, are especially suitable foruse in the method of the present invention because they are readilyformed into easily handled, tough, flexible sheets having a uniformquantity of explosive per unit area. The tough, flexible sheet-likenature of these compositions is advantageous because they handle easily.Moreover, the uniform distribution of explosive therein results in auniform detonation velocity and insures consistency of detonation, bothof these features being desirable in the present method. However, theuse of other detonating explosives is equally feasible; for example,those cohesive, gelatinous detonating explosives based uponnitroglycerin, for example, blasting gelatin, can be formed intosheet-like uniformly dense explosive layers. Noncohesive solid andliquid high explosives may also be used by maintaining them in asuitable container. Castable explosives, for example those like amatol(TNT-ammonium nitrate mixture) or cy-clotol (a TNT-RDX mixture),naturally may be readily cast into plate-like or annular charges for usein the instant process. The quantity of explosive used will depend uponthe Hugoniot elastic limit of the stress-containing metal object, thelevel of residual stresses in the object, the amount of reduction ofstresses desired, the thickness of the object, the particular explosivecomposition used, and the composition and thickness of the buffer layer,among other factors. With a particular set of conditions, increasing thequantity of explosive increases the pressure of the shock entering theshock-wave-transfer medium. This may be desired for treating objects ofhigher stress levels or thicker objects, and can be used provided theHugoniot elastic limit is not exceeded by five times or more. Explosiveloadings which are too high for a specific set of conditions can becompensated for by a thicker and/or denser buffer layer or use of alower-velocity explosive. In general, for obtaining the pressuresrequired in the common metals with the preferred shock-wave-transfermedia and explosives in the form of a layer and in the velocity range of5000-7000 meters per second, explosive loadings of about 2 to 4 gramsper square inch are used for metal pieces about 3 inches thick. Theexplosive loading may be increased for thicker metal pieces and reducedfor thin metal pieces, and generally falls within the range of 1 tograms per square inch.

The explosive can be initiated by any conventional initiating device,e.g., blasting cap, exploding wires, detonating cord, line-wavegenerator, surface-wave generator or any suitable combination thereof.The location of initiation on the explosive charge can be at a point,e.g., at a point along an edge, a corner, or in the center of a layer,along a line such as an edge of a layer, or simultaneously over theentire surface of a layer opposite the surface adjacent the layer of ashock-wave-transfer medium. Pointor line-initiation produces an obliqueshock wave at lower pressures and for this reason is preferred in thepresent process. For example, as is shown in the case of the plate-likelayer shown in FIGURE 1, a linewave generator can be attached at onelinear boundary of the layer and the line-wave generator initiated,e.g., with a blasting'cap. The line-Wave generator shown in FIG- URES l,2, and 3 as well as others which can be used to initiate an explosivelayer simultaneously at a plurality of points along a line are describedin detail in U.S. Patent 2,943,571. When the metal object to bestressrelieved is in the form of a cylinder or a rod, the explosivelayer around the cylinder or rod can be initiated by afiixing one ormore line-wave generators along one circular edge of the explosivelayer, or a conical explosive charge filled with an inert material alongone circular edge, and initiating with one or more blasting caps.

The particular surface of the metal object at which the pressure waveenters the object will depend on the configuration of the metal objectand generally will be decided on the basis of convenience and economy.For example, in the case of a metal plate, this surface will be thesurface of greatest area, i.e., the horizontal surface. Such anarrangement is desirable for the following reasons: (1) as large an areaas possible can be treated in one operation; and (2) the pressure waveis caused to pass through the region of minimum thickness in the object.In the case of metal objects having irregular surfaces, generally italso will be desirable to have the wave enter at the surface which willallow treatment of as large an area as possible and which will present aminimum thickness through which the wave must travel. However, in thecase of irregularly shaped objects, convenience of maintaining theobject in a stable position becomes a determining factor. For example,the cavity die forging shown in FIGURE 4 was more convenientlymaintained in a stable position when placed flat as shown in FIGURE 3,than if it were allowed to stand on its curved surface. In the case ofmetal objects whose surface is completely or largely curved, e.g.,spheres or rods, the buffer and explosive layers will surround thecurved surface so that the largest possible area can be treated in oneoperation.

In most cases an explosive layer will be positioned essentially parallelto the metal object. This means that the explosive layer may be parallelto the horizontal surface of a flat even-surfaced object or to thehorizontal axis .of a flat uneven-surfaced object, e.g., the object ofFIGURE 4. However, in some cases, e.g., those in which the metal objectis so shaped that parallel positioning would locate some regions of itssurface considerably closer to the explosive than others, it may bedesirable to position the explosive layer at an angle to the horizontalsurface or axis of the metal object. Such positioning is within thescope of the present process provided that the layer of explosive is ofsuflicient dimensions to induce a pressure wave which will pass throughthe object in all of the regions where stress relief is desired.

As is illustrated in the drawings and examples, when the metal object tobe subjected to the explosively induced pressure wave has portions ofsignificantly less thickness than other portions, it may be desirable tosupport the thinner portions on that side of the object which is awayfrom the explosive, i.e., the side which the wave reaches last. Forexample, in the object shown in FIGURE 4, the web portion, orreduced-thickness portion, of the forging is supported by filling thearea between flanges on the side opposite the explosive with a materialhaving strength sufficient to prevent deformation. Preferably, thematerial will be a solid, e.g., a metal such as steel or lead, orconcrete. For convenience it is preferred to use Woods alloy since thismetal, because of its low melting point, can easily be applied in themolten form.

The present process is applicable to any metal which containsunfavorable residual stresses. Metals, both ferrous and nonferrous,which commonly require stress relief in commercial practice and to whichthe present process can advantageously be applied include, for example,the steels, e.g., carbon steel, carbon-molybdenum steel,chromium-molybdenum steel, chromium stainless steel, chromium-nickelstainless steel, and weldments of dissimilar steels; aluminum alloys;copper alloys; lead alloys; magnesium alloys; nickel alloys; tin alloys;and zinc alloys. While applicable to all metals, both alloyed andunalloyed, the process finds particular advantage when applied to thosemetals which cannot be thermally stress-relieved without accompanyingadverse effects on mechanical properties, i.e., metals which undergo adeterioration in mechanical properties when subjected to thetemperaturetime conditions required for thermal stress relief. Includedamong such metals are the precipitation hardening alloys, i.e., alloyswhich harden by precipitation at room temperature or above from asupersaturated solid solution of the alloy obtained by rapid cooling ofa hot solution thereof. This phenomenon occurs, for example, in the caseof certain alloys of aluminum, copper, magnesium, or nickel, and thesteels. The process of this invention is particularly useful withprecipitation-hardened alloys of aluminum with minor (e.g., 20% or less)amounts of at least one of copper, magnesium, zinc, manganese, iron,chromium, nickel, titanium, and silicon. As is shown in the subsequentexamples, the precipitation-hardening aluminum alloys acquire noundesirable properties as a result of stress-relief treatment accordingto the process of this invention.

The following examples serve to illustrate specific embodiments of theprocess of the present invention. However, they will be understood to beillustrative only and not as limiting the invention in any manner. I

The residual stresses reported in the examples are measured by themethod of Siebel and Pfender as described in Metals Handbook, 1955Supplement, American Society for Metals, Cleveland, Ohio, page 96.Blocks are cut out of a section of the metal object and are measuredbefore and after sectioning. The lengths are introduced into standardelasticity equations which are used to calculate the stresses.

Example 1 A stress-relief assembly as depicted in FIGURE 1 is erected.The metal object 1 is a Z-inch-thick plate of aluminum alloy, 6 incheswide and 12 inches long. The alloy, which is of the precipitationhardening type, has the following composition: 93.6% aluminum, 4.4%copper, 0.8% silicon, 0.8% manganese, and 0.4% magnesium. The residualstresses in the plate amount to 15,000 p.s.i. The explosive layer is anexplosive sheet, 6 inches wide and 12 inches long, comprising a blend ofPETN in a 50/50 mixture of butyl rubber and a thermoplastic terpeneresin (mixture of polymers of fl-pinene having the formula (C H theexplosive load being 2 grams per square inch. This explosive sheet isdescribed in detail in U.S. 2,999,743. The explosive sheet ischaracterized as strong, flexible, waterproof, uniform in density, andnonresilient, the composition detonating at a uniform velocity of about7000 meters per second. A line-wave generator (6-inch equilateraltriangle) of the type shown in FIGURE 2B of US. Patent 2,943,571 istaped to the shorter edge of the explosive sheet in such a way thatthere is uninterrupted contact between the line-wave generator and theexplosive sheet. A commercial detonator (a No. 8 electric blasing cap)is fastened to the generator at the angle opposite the base contactingthe explosive sheet. The metal plate is immersed in a wooden boxcontaining sufficient water so that a 1-inch layer of water covers theplate. A A-inch-thick piece of wood of such dimensions as to fit snuglyin the box, parallel to the bottom of the box, is placed over the waterlayer to P ovide a means of supporting the explosive sheet. Theexplosive sheet and affixed initiating means are afiixed to the bottomsurface of the wood support so that the explosive contacts the waterlayer and so that the edges of the sheet are in alignment wih the edgesof the metal plate. The blasting cap is initiated by application of anelectric current, causing detonation of the line-wave generator and theexplosive sheet, and introducing into the metal plate a transientdisturbance at a pressure of about 10-20 kilobars determined asdescribed hereinbefore (from shock velocity measurements in water).Residual stress measurements made on the aluminum alloy plate after thisstress-relief treatment show that the stress has been reduced to psi.The reduction in thickness of the plate after such treatment amounts toonly 0.7%.

Similar results are obtained when the experiment is repeated under thesame conditions except that the depth of the water layer is 0.25 or 0.5inch.

When the same explosive layer described in this example is placed indirect contact with an aluminum alloy plate of the same composition andsize as that used in this example, the plate having a compressive stresslevel of 17,000 p.s.i., and the explosive is initiated in the same way,so as to introduce into the aluminum plate a shock wave at a pressure ofabout kilobars, the plate after shocking has a tension stress level of40,000 p.s.i. Thus, explosive hardening techniques, which employpressures of about 100 kilobars or more, have a completely oppositeetfect when contrasted with the present process, which markedly reducesstresses in metals.

Example 2 An assembly as depicted in FIGURE 3 is erected. In this case,the metal object to be stress-relieved is a cavity die forging, such asthat depicted in FIGURE 4, and has the following composition: 90.2%aluminum, 5.5% zinc, 2.5% magnesium, 1.5% copper, and 0.3% chromium. Thesample has been solution heat-treated and is at an unstable temper. Theflange portion of the forging varies in thickness from 2.215 inches to2.845 inches, depending on the location of measurement. The explosivelayer in this instance is an explosive sheet comprising a blend of PETN(35%) and red lead (50%) in a binder (15%) consisting of 50% butylrubber and 50% of a thermoplastic terpene resin (mixture of polymers offl-pinene having the formula (C H and described in detail in US. Patent3,093,521. This composition has a detonation velocity of about 5000meters per second. The explosive load of the sheet is 2 grams per squareinch. The thickness of the Water layer is one inch. Initiation of theexplosive layer is by means of a line-wave generator and blasting cap asdescribed in the preceding example. Before the assembly is completed,the web portion (see 1b of FIGURE 4) of the forging is supported on theside of the forging which is away from the explosive by filling the areabetween flange portions (see 1a of FIGURE 4) on the side opposite theexplosive with Woods alloy, in a manner similar to that shown in FIGURES5 and 6. To facilitate removal of the Woods alloy subsequent to thestress-relief treatment, the area to be filled is lined with aluminumfoil prior to the filling. After the explosive has been initiated andthe shock wave has passed through the forging at an estimated pressureof 10-20 kilobars, the Woods alloy sections are removed by simplylifting them out manually. After treatment, the flange portion of theforging varies in thickness from 2.206 inches to 2.837 inches. Theaverage compressive deformation is only 0.26% (less than the deformationrequired by the cold compression method of stress relief). The forgingis then artificially aged to the T6 temper and cut up to determinetensile properties and residual stresses. The residual stress range(dilference between the maximum and minimum residual stress as seen inthe stress distribution plot for the explosively shocked forging) is 5-6k.s.i. (kips per square inch; 1 kip=l000 p.s.i.) as sampled at variouslocations in the flange portions. In contrast, a similar forging (asquenched) which has not been subjected to stress relief treatment has aresidual stress range of 25.6-

Kips per square inch Cold compression 2.5-7.5 Thermo-mechanical 4-9.5

The tensile properties of the forging are not adversely affected by theexplosive treatment. The explosively stress-relieved forging has atensile strength of 78,900- 87,400 p.s.i., yield strength of68,00077,600 p.s.i., and elongation in 4D of 11-14% (all above minimumvalues for die forgings of the composition used).

I claim:

1. A process for relieving stresses in a metal object which comprisespositioning said object in spaced relationship to a detonatingexplosive, placing a shock-wave-transfer medium therebetween, anddetonating said explosive, the pressure wave resulting from saiddetonation of said explosive and traversing said metal object having anintensity which exceeds, but is less than about five times, the Hugoniotelastic limit of said object.

2. A process of claim 1 wherein said shock wave transfer medium isliquid.

3. A process of claim 2 wherein said explosive is a layer of explosiveof the general contour of the juxtaposed surface of said object.

4. A process of claim 3 wherein said metal object is aprecipitation-hardened alloy.

' 5. A process for relieving stresses in a precipitation hardenedaluminum alloy object which comprises positioning said object in spacedrelationship to a layer of detonating explosive, the space between saidobject and said explosive being filled with water, and detonating saidexplosive, the pressure wave resulting from detonation of said explosiveand traversing said object having an intensity of about from 1 to 30kilobars.

6. A process of claim 5 wherein said explosive has a detonation velocityof about from 5,000 to 7,000 meters per second and the thickness of thelayer of water between said explosive and object is about from A to 1inch.

References Cited by the Examiner UNITED STATES PATENTS 1,891,234 12/1932Langenberg 1484 2,703,297 3/1955 Ma'cLeod 148-4 FOREIGN PATENTS 879,93310/ 1961 Great Britain.

OTHER REFERENCES DAVID L. RECK, Primary Examiner.

C. N. LOVELL, Assistant Examiner.

1. A PROCESS FOR RELIEVING STRESSES IN A METAL OBJECT WHICH COMPRISESPOSITIONING SAID OBJECT IN SPACED RELATIONSHIP TO A DETONATINGEXPLOSIVE, PLACING A SHOCK-WAVE-TRANSFER MEDIUM THEREBETWEEN, ANDDETONATING SAID EXPLOSIVE, THE PRESSURE WAVE RESULTING FROM SAIDDETONATION OF SAID EXPLOSIVE AND TRAVERSING SAID METAL OBJECT HAVING ANINTENSITY WHICH EXCEEDS, BUT IS LESS THAN ABOUT FIVE TIMES, THE HUGONIOTELASTIC LIMIT OF SAID OBJECT.